Optical components can be used in optical systems to alter the state of visible light in a predictable and desired manner, for example in display systems to make a desired image visible to a user. Optical components may also be used as e.g. moulds for making other optical components. Optical components can interact with light by way of reflection, refraction, diffraction etc. Diffraction occurs when a propagating wave interacts with a structure, such as an obstacle or slit. Diffraction can be described as the interference of waves and is most pronounced when that structure is comparable in size to the wavelength of the wave. Optical diffraction of visible light is due to the wave nature of light and can be described as the interference of light waves. Visible light has wavelengths between approximately 390 and 700 nanometers (nm) and diffraction of visible light is most pronounced when propagating light encounters structures similar scale e.g. of order 100 or 1000 nm in scale. One example of a diffractive structure is a periodic diffractive structure. Periodic structures can cause diffraction of light which is typically most pronounced when the periodic structure has a spatial period of similar size to the wavelength of the light. Types of periodic structures include, for instance, surface modulations on a surface of an optical component, refractive index modulations, holograms etc. Herein, a “diffraction grating” (or simply “grating”) means any (part of) an optical component which has a diffractive periodic structure. A diffraction grating has a grating period, which is the distance over which its structure repeats. When propagating light encounters the periodic structure, diffraction causes the light to be split into multiple beams in different directions. These directions depend on the wavelength of the light thus diffractions gratings cause dispersion of polychromatic (e.g. white) light, whereby the polychromatic light is split into different coloured beams travelling in different directions.
When the period structure is on a surface of an optical component, it is referred to a surface grating. When the periodic structure is due to modulation of the surface itself, it is referred to as a surface relief grating (SRG). An example of a SRG is uniform straight grooves in a surface of an optical component that are separated by uniform straight groove spacing regions. Groove spacing regions are referred to herein as “lines”, “grating lines” and “filling regions”. The nature of the diffraction by a SRG depends both on the wavelength of light incident on the grating and various optical characteristics of the SRG, such as line spacing, groove depth and groove slant angle. SRGs have many useful applications. One example is an SRG light guide application. A light guide (also referred to herein as a “waveguide”) is an optical component used to transport light by way of internal reflection e.g. total internal reflection (TIR) within the light guide. A light guide may be used, for instance, in a light guide-based display system for transporting light of a desired image from a light engine to a human eye to make the image visible to the eye.
In the case of a waveguide-based display system, different gratings forming part of the same waveguide may serve various functions. Waveguide-based display systems typically comprise a light engine, which collimates light of an image into collimated input beams which form a virtual version of that image at infinity. The input beams may be directed towards an incoupling grating of the waveguide, which is arranged to couple them into the waveguide at angles which are sufficiently steep to cause TIR of the incoupled beams within the waveguide. An outcoupling (exit) grating on the waveguide may receive the incoupled beams internally and diffract them outwardly in directions that match the input beams (so that they form the same virtual version of the image). A user's eye can then reconstruct the image when looking at the exit grating. Usually, the exit grating is also arranged to provide beam expansion of the outputted beams so as to provide an eyebox of increased size compared with viewing the light engine directly. Intermediate grating(s) of the same waveguide may provide additional beam expansion to further increase the size of the eyebox.
For some such waveguide grating arrangements, the incoupling, outcoupling and (where applicable) intermediate grating(s) will only manipulate the image light as intended if their various gratings are oriented relative to one another in a specific manner. Deviation from this intended orientation can cause degradation of the final image as perceived by the user. When such waveguides are manufactured in bulk for incorporation in different waveguide display systems, each should preserve these specific relationships to avoid degrading the quality of the final display systems. Other types of optical component with various applications may also comprise different gratings where it is desirable for the relative orientation of those gratings to match a desired value as closely as possible.